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I want more focal length, but via what route?


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I hate myself for admitting it, but I have become hooked on astrophotography. Currently, my imaging rig consists of a Vixen SXD2, an Askar FRA400, a ZWO ASI533 camera and an ASIair Pro + little guide scope. It's great; it works, but what I really crave is those tiny little galaxies  or groups like Stephan's Quintet. I need more focal length. I have three options floating round in my mind. 

1. I have a Takahashi Mewlon 180. Visually, it's beautiful and it seems to hold collimation perfectly. It would seem sensible to use this if it's a reasonable solution. From what I've read though, it might be a bit of a pain when it comes to astrophotography; the focuser moves the mirror, it's not flat and I'd need a £350 reducer/corrector to make it work to any decent level. There are then issues with mounting any other gear as I can't find any tube rings etc and there isn't even a finder shoe on it. I suppose a very long dovetail on the bottom could give me enough mounting space. 

With the reducer, I'd have an f9.8 with 1760mm focal length. 

2. The obvious solution really, an RC6 such as the Stellalyra. It has a decent focuser, plenty of mounting options, and doesn't cost a lot more than a reducer for the Tak. It would also give me two focal length options, both a lot less onerous than the Tak but still enough to get me "closer" those targets I crave. My main concern is collimating an RC; it's not that hard is it? The astronomy tools site suggests it's a good match for my ASI533 camera. 

3. Probably silly, but how about a long focal length refractor like a Stellamira 80mm f10? No collimation needed there (hopefully), but perhaps not enough aperture to pull in those fainter objects and a bit restricted on the focal length....if it would work at all that is. I do like refractors!

My heart says make use of the Mewlon, my head says get an RC6. Any thoughts?

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57 minutes ago, osbourne one-nil said:

….My main concern is collimating an RC; it's not that hard is it?….

Hi, I read this bit and recalled my own tribulations regarding RC collimating, about which I had a few gripes on this forum. It’s a subject often discussed on the forums.

However, once you get used to the nuances of RC collimation I eventually found that it’s really very simple, and I feel a little embarrassed at my earlier thread. It also doesn’t require specific tools that some websites suggest. The great thing, though, is the the mirrors stay put and my RC8 simply doesn’t need collimating despite being taken on and off the mount and shifted around.

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4 hours ago, osbourne one-nil said:

. It's great; it works, but what I really crave is those tiny little galaxies  or groups like Stephan's Quintet.

Probably best thing to do is to try to understand limits of DSO imaging in terms of achieved sharpness and detail.

Many people have this idea that they want to "get in close" to galaxies. They want big image of small galaxy. In most cases - you can just take small image of the same galaxy and enlarge it in software - you'll get the same result as spending money and time in trying to image it.

There is a limit to what can be achieved and that limit depends on several factors - seeing, mount performance and optics.

In most cases, that limit is at about 1.5"/px. If you have larger scope and very good mount - it can be as low as 1.2"-1.3". In ideal conditions you might be able to reach 1"/px - but that happens in like 1% of the time - only with best mounts and large aperture scopes.

You have 533 camera that has 3.75µm pixel size. Your baseline focal length is about 520mm. With that focal length you have 1.5"/px sampling rate. You can double that focal length or triple it - then you'll bin your data x2 or x3 to recover lost SNR and you'll simply reduce your FOV.

Having said all of the above, if you want good and affordable telescope and you don't shy away from mirrored systems - take a look at this scope:

https://www.firstlightoptics.com/ts-telescopes/ts-photon-6-f4-advanced-newtonian-telescope-with-metal-tube.html

It will provide you with plenty of aperture and 600mm of focal length (somewhat longer but still within limits - 1.3"/px sampling rate).

It will be a bit fiddly to get it going - collimation of such scope is not easy, you need to purchase good coma corrector for it as well, and you need to shield from stray light to avoid light leaks (look at back of the scope - you'll need to put some sort of shielding over mirror as it has holes around it so light can leak in - similarly add a dew shield type extension to ota at front side).

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Seconding what @vlaiv stated above. I prefer galaxies the most, then star clusters, anything else and then nebulae. So I also tackled the long reach scope options when moving from analytical astronomy to imaging/pictures. I used to use larger RCs for infra-red imaging and photometry - they had reach but were corrected and purely mirror systems. To image galaxies and galaxy clusters, I went for speed instead at a resolution of 1.8 "/px, rather than reach. I was a little tired of waiting week to get enough data, but also realised that the pixel scale difference was not an issue and digital rescaling works just fine. I have a workhorse Intes MN56 at 762 mm focal length which is 1"/px with the same camera. Both are good and close enough that I can mix data between them if I wanted. 

The f/5, 1000 mm or so newtonian (imaging type) is a sweet spot, pushing the real resolution capability only a little and relatively easy to deal with collimation/optical axis alignment compared to f/4 or faster and any aperture RC. The fastish newts, even at the 450-600 mm focal length give wider fields, but lots of signal and good resolution that you can digitally zoom into without worry. I think these are the best option and give you a chance for nebulae and galaxy groupng that 1000 mm+ cannot really do without mosaicing.

Great examples around astrobin. Here is some images to think about from Michael Deger where at least he has broken down the images into those taken with the larger Lacerta newtonian, but also a relatively fast 4.5" newtonian. You can see this in the image lists tab, which is a handy way to get a sense of galaxies, groups etc on both systems.

https://galaxyphoto.de/en/astrograph/

The TS photon scope recommendation is good, with some choices for higher grades of focuser or other parts if needed from TS. For what it is worth, I am happier with 420 mm focal length and speed/signal at 1.8 "/pixel and can safely zoom in if I wanted to. It is a nice balance in my opinion.

 

 

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Thanks all…I’ve been without internet all day so this is nice to come back to. That all makes sort of sense and I accept I was being very simplistic. 
 

I guess I need to consider my mount’s capacity. 
 

Would now be a good time to own up to always having craved an R200SS? I’m not sure why…perhaps I’m a Vixen guy! 
 

I shall ponder on your advice. Thanks again. 

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11 hours ago, osbourne one-nil said:

Thanks all…I’ve been without internet all day so this is nice to come back to. That all makes sort of sense and I accept I was being very simplistic. 
 

I guess I need to consider my mount’s capacity. 
 

Would now be a good time to own up to always having craved an R200SS? I’m not sure why…perhaps I’m a Vixen guy! 
 

I shall ponder on your advice. Thanks again. 

Well you do have a Tak Mewlon. Frankly, I would love to try one of those. But I went for speed first, with the realization that I can push the 'zoom' to a certain level without changing scope, unless I opted for a very large aperture for the sake of f-ratio, while sacrificing some of the resolution it could be capable of in ideal skies. They are also way beyond my finances though. What is the focal length of the mewlon with the reducer/flattener?

The Vixen's are nice, I am also a fan of the 'hospital colored' scopes. I used the older VC200L some years ago, and would like to have the new version since I like their approach to the optical design. However, the fast wider field scope with 3.76 micron pixels seems to suit OK.

The coma corrector for the R200SS reduces it down to below f/4, something to consider, giving about 800 mm fl (afaik), which is a nice combination.

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I suppose the logical thing to do is try the Mewlon as it is and see what I think; ultimately if I’m having fun and the shots are nice, I’m the only judge! I’ve attached my Fuji XT3 before and shot the odd thing through it, unguided, and even then I went “wow” so if I can suss a bit of guiding then what’s to lose?!

With a reducer it’s 1750mm (or thereabouts) so perhaps a dovetail long enough to mount a guidescope and ASIair pro is the starting point. Perhaps the 400mm Askar might be a more suitable guidescope at that focal length. I guess I have a lot of options to try before committing to another scope? I just want to have fun and get some photos the kids go “cool” to so they don’t think I’m a total geek. 

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